It is well known that faults in optical fibres can be located by an optical time domain reflectometer (OTDR). An OTDR launches a pulse of light into an optical fibre, and backscattered light is monitored for abrupt changes indicative of a loss or fault. The distance of the loss or fault from the launch end of the fibre can be determined from the time interval between launch and return of the backscattered peak. Once a period of time sufficient to receive all detectable backscattered light has passed, a further pulse may be launched into the fibre. The pulse width may be varied for different dynamic range or resolution requirements. Thus, for a given amplitude, an increase in the pulse width enables a greater length of fibre to be monitored, that is to say it increases the dynamic range of the OTDR. The dynamic range of an OTDR is the loss after which an event, backscatter or reflection can still be detected.
The OTDR is an extremely useful form of optical test equipment since, from connection to a single end of an optical fibre network, the location of losses and reflections can be determined, and their amplitude measured, to a high degree of accuracy. For loss measurements, both point-losses and end-to-end fibre or network losses can be measured. In duplex networks, the amplitude of any reflections is important, since these may cause crosstalk. In some known schemes it is possible using wavelength division multiplexing (WDM) techniques, to take these measurements at a particular wavelength i.e. 1650 nm whilst the network is carrying data at another wavelength i.e. 1550. However, one drawback in using WDM techniques is the required use of relatively expensive WDM couplers. On the other hand, inexpensive couplers tend to have an associated signal loss that is not acceptable for many OTDR applications.
It is an object of this invention to provide an OTDR that will obviate the use of WDM couplers.
It is a further object of the invention to provide an inexpensive device for testing an optical waveguide in the absence of traffic.
More generally, it is also an object of the invention to provide a device and method for inserting light signals into, or receiving light signals from, an optical path while temporarily interrupting a normal flow of signals in the path, and to provide a method for performing this same function.